ChengPrusoff

The Cheng–Prusoff relationship is a foundational concept in pharmacology and enzymology that connects the observed inhibitory concentration of an agent to its intrinsic binding affinity. In the case of competitive inhibition, the relationship expresses that IC50 (the inhibitor concentration producing 50% inhibition) depends on substrate concentration and the enzyme’s affinity for the substrate. The standard form is IC50 = Ki(1 + [S]/Km), which can be rearranged to Ki = IC50 / (1 + [S]/Km). This allows researchers to estimate the true inhibition constant Ki from measurements made at a given substrate concentration.

History and usage: The relationship was introduced by Y. C. Cheng and W. H. Prusoff in 1973 and has since become a widely cited tool in drug discovery and biochemical assays. It provides a way to compare inhibitor potency across experiments conducted at different substrate levels, by correcting IC50 values to a substrate-independent Ki.

Assumptions and limitations: The Cheng–Prusoff equation assumes competitive, reversible inhibition, a single binding site, and rapid equilibrium among enzyme, substrate, and inhibitor. It relies on known and constant [S] and Km. If inhibition is noncompetitive or mixed, or if substrate concentrations are far from Km, or if enzyme concentration is non-negligible, the simple form may not apply and alternative relations or direct Ki determinations should be used.

Applications: It is commonly applied in enzyme kinetics and pharmacology to standardize and interpret inhibitor potency, particularly in comparative studies and during early stages of drug development.